Ambient Occlusion artifacts on Intel HD4000

[matthewharvey]

Turning on Ambient Occlusion causes the following artifact:

I'm assuming that those artifacts underneath the kart and at the upward edges of the road aren't supposed to be there.

The console continuously prints the following lines while this is going on:
Mesa: User error: GL_INVALID_FRAMEBUFFER_OPERATION in glDrawElements(incomplete framebuffer)
Mesa: User error: GL_INVALID_FRAMEBUFFER_OPERATION in glClear(incomplete framebuffer)

This is on an Intel HD4000 using the latest code and assets. I tried it on both a 'next gen' and a 'standard' track with the same result in both. I also tried turning other graphics features on and off, but the only thing that seemed to affect the artifacts was Ambient Occlusion.

[matthewharvey]

Just to clarify, the issue in the provided image is the corruption of the ground underneath the kart and to the sides of the road. I noticed that it kind of looks like something is going on in the sky as well, but that seemed independent of the graphics settings, and may or may not be an issue for all I know. :D

[auriamg]

@vlj : any idea?

[vlj]

Not really... Does it work with shadows and Global Illumination disabled ?

[vlj]

Please provide stk log too

[matthewharvey]

After disabling shadows and Global Illumination the issue is still present, exactly as described above.
I have one correction to make though. The console prints about invalid framebuffer are un-related.

I wasn't totally sure what you meant by stk log, but I saw some stdout.log files in ~/.config/supertuxkart. So I just redirected my session to a file and pasted that here:
http://pastebin.com/392uYhnp
(note that this doesn't include Mesa errors printed to the console because I have set MESA_DEBUG. There weren't very many and they looked very unrelated)

Some more details:
This is present on the latest mesa code and in the 10.3.2 release.

[hiker]

Is this still an issue in our second beta?

[matthewharvey]

Instead of testing with the beta, I tested against the latest code. Let me know if you think it's relevant to use the beta package instead. Also, I tested it on both the latest code and the 10.3.2 release of Mesa, with the same results on each.

It's not as pronounced anymore, but it's still obvious and distracting.

Also, I've noticed that it is highly dependent on camera distance. When the camera is far away from the kart, it disappears totally, and as the camera comes closer it becomes more pronounced. This is true for the artifacts under the kart and the ones that sometimes appear on other objects.

[hiker]

@vlj any ideas?